An optical heterodyne detection communication system in which a coherent light optical fiber is used has an advantage that a long distance and high density transmission can be performed because a receiving sensitivity is remarkably high, and a frequency utilizing efficiency is also high, as compared to a direct detection communication system in which an intensity of light is modulated. The optical heterodyne detection communication system is described in a report entitled "a coherent light optical fiber transmission demodulation technology--FSK heterodyne detection" in "The Practical Report, Vol 31, No. 12, 1982" published by Nippon Telegraph and Telephone Corp.
In the coherent light communication, signal light supplied from an optical transmitter is coupled with light supplied from a local oscillation light source to provide combined light which is then received in an optical detector. As a result, beat signal corresponding to the difference of frequencies between the signal light and the local oscillation light is produced in the optical detector as electric signal of intermediate frequency. Then, the intermediate frequency signal is demodulated to produce base band signal. For the purpose of stabilizing frequency and level of the intermediate frequency signal which tend to fluctuate due to the fluctuation of frequency difference between the signal light and the local oscillation light and to the incoincidence of polarizations thereof, the frequency and polarization of the local oscillation light are controlled to synchronize with the fluctuation of the center frequency and polarization of the signal light. Such a control for the stabilization of the frequency and level of the intermediate frequency signal is also adopted in a freqnecy division multiplexed transmission system. That is, the frequency sweep of local oscillation light is performed to pull beat signal of a predetermined channel into an intermediate frequency band, and the above mentioned frequency and polarization control is then performed in a case where the predetermined channel is selectively received from frequency division multiplexed signals.
In the optical heterodyne detection communication system, however, there is a disadvantage that, in a case where frequency division multiplexed signals of, for instance, two channels having polarizations orthogonal to each other are received, beat signal of a first channel to which a second channel is changed-over is not detected, thereby making it impossible to pull the beat signal into intermediate signal band, even if the frequency sweep of local oscillation light is performed to change-over from the second channel to the first channel. In regard to this disadvantage, a probability of not detecting the beat signal is increased in proportional to the number of channels.